Enclosure for electronic modules

ABSTRACT

A Faraday shield minimizes the leakage of electromagnetic interference (EMI) and radio frequency interference (RFI) of a maximum frequency and corresponding wavelength that emanates from electronic components contained within the shield. The top and bottom each contain apertures that are dimensioned to effectively block the escape of EMI and RFI from the shield and to permit the flow of air to dissipate heat without generating acoustic noise. The thickness of the top of the shield is at least one half of the diagonal length across the largest of the apertures located in the top. The length of the longest aperture side is less than one fourth of the wavelength of the maximum frequency of EMI and RFI contained by the shield. Corresponding relationships exist between the apertures located in the shield bottom and its thickness.

FIELD OF THE INVENTION

The present invention relates to enclosures for receiving and housingvarious electronic modules or cards and which acts as a Faraday shieldto prevent emissions of electromagnetic interference (EMI) and radiofrequency interference (RFI). Counterbalancing the desire to preventemissions is the thermodynamic requirement to dissipate heat, since theelectronic modules or cards create heat during operation. Accordingly,this type of enclosure must be adequately vented for heat dissipationwithout having the movement of air create undesirable noise. Inaddition, since the electronic modules or cards may be frequentlyremoved and reinserted, contact between the module or card and theenclosure should not cause wear to the enclosure resulting in damage orthe release of contaminants. Furthermore, during insertion of the moduleor card any static electric charge on the module or card must bedissipated. Finally, the module or card must be prevented from beingwithdrawn while power is being applied to the module or card to preventdamage or interruption to the operation of other modules or cards.

BACKGROUND OF THE INVENTION

In known enclosures for receiving and housing electronic modules orcards containing electronic components, the RFI/EMI emissions arecontained by enveloping the enclosure in a sheet metal shroud andplating the plastic components of the housing. The sheet metal shroudtypically has a plurality of small holes for ventilation and eithercompletely covers the enclosure or at least covers substantial portionsof the enclosure. This sheet metal shroud is effective in preventingRFI/EMI emissions from the electronic components but adds substantialweight to the overall enclosure. In addition, if aluminum foil is usedinstead of sheet metal, the overall material cost is increased.

Known enclosures that provide the necessary Faraday shield through theuse of sheet metal typically have a plurality of small holes forventilation, typically, one-eighth of an inch in a material with thethickness of approximately one-sixteenth inch. Any large openings topermit air flow also permit the release of RFI/EMI emissions. Knownenclosures use fans to move air through these small openings todissipate the heat generated by the electrical components. However, inorder to adequately cool such known systems, the volume of air flowthrough the relatively small apertures create an air turbulence noise.When the enclosure is placed in an otherwise quiet work environment, theair flow noise is a source of irritation and distraction to workers inthe area.

In a typical enclosure, the electronic modules or cards are insertedinto guides or channels which orient the modules or cards within theenclosure. The frequent removal and reinsertion of the modules or cardswithin the guides of the enclosure cause wear to the surface of theguides. The wear of the guides results in the release of small particlesof conductive material which can settle on the modules or cards andcause damage to sensitive electronic components and connections.Furthermore, the removal of material from the guides can create a holein the Faraday shield and the leakage of emissions.

When a module or card is inserted into the enclosure, electrostaticdischarge (ESD) resulting from a static electric charge build-up on themodule or card can be transferred throughout the entire system, causingdamage to sensitive components. In order to eliminate damage to systemcomponents from ESD, known methods include static pads that the operatorstands on during assembly and various sprays to remove staticelectricity. These methods are relatively ineffective and requireoperator cooperation. The specification for the IEEE Futurebus, which isdefined by IEEE P896.2, requires some form of positive discharge ofstatic electricity between the module or card and the enclosure. Ofcourse, the Futurebus specification and other relevant specifications,such as the IEEE Standard for a Metric Equipment Practice forMicrocomputers Coordination (IEEE P1301) do not prescribe any physicalstructure to achieve the desired static discharge.

In known enclosures, the modules or cards if withdrawn while power ispresent can cause a power surge resulting in damage to other modules orcards or a disruption in their operation. If a module or card isinserted into an enclosure that has power being supplied to othermodules or cards, damage because of high current inrush is possible. Inorder to prevent the withdrawal of modules or cards, various electroniccircuits or mechanical locking devices have been developed. Thecomplexity of the mechanical locking devices increases the cost ofmaterials and assembly of the enclosure.

SUMMARY OF THE INVENTION

The present invention is an enclosure for receiving and holding aplurality of electronic modules or cards and which provides a Faradayshield to prevent EMI/RFI emissions. The enclosure has an upper chamber,a card cage subtending the upper chamber and a bottom chamber subtendingthe card cage. The card cage has plastic molded top and bottom cardguides which are mirror images of each other. Each card guide has anelectrolysis nickel over electrolytic copper plating to provideconductivity. Each card guide has a plurality of molded slots orchannels to receive the electronic modules or cards. A grid subtends thechannels and has a plurality of openings. The size of the openings areselected to balance the functional requirements of suppressing EMI/RFIemissions and allowing for adequate air flow to dissipate heat withoutcausing acoustic noise. The sides of the card cage are formed out ofsheet metal or similar material to form a part of the Faraday shield.The back of the card cage has a sheet metal or similar material rearpanel and a backplane assembly for providing electrical connection forthe electronic modules or cards. The front panel of the card cagecomprises a plurality of module bulkheads which are plated as describedabove and form the front portion of the electronic modules or cards. Thetop and bottom card guides, the sides, rear panel and front panel areconnected together in any manner well known in the field to complete theFaraday shield. The upper chamber contains at least one fan to draw airfrom the bottom chamber through the card cage to dissipate the heatcreated by the electronics.

Each slot or channel in the top and bottom card guides of the card cagecomprises two substantially parallel spaced apart wall portions and abottom portion. In the bottom card guide one of the wall portions has aslight recess or offset. An anti-static clip is partially positionedwithin the recess of the wall portion. The clip has a saddle portionwhich fits over the wall portion and a bowed portion which extends intothe channel and contacts a conductive static strip formed along thebottom of each electronic module or card. When the bowed portion of theclip contacts the electronic module or card static strip, the bowedportion compresses, which forces its distal end to extend further intothe recess in the wall thereby assuring an electrical contact betweenthe anti-static clip and the module or card to dissipate the staticelectric charge accumulated on the module or card. At the same time, thebias pressure from the clip aligns the module or card within thechannel. The recess restrains the clip from horizontal movement alongthe slot wall when the electronic module or card is inserted. The topcard guide of the card cage is an identical mirror image in structurebut does not have an anti-static clip mounted in the same fashion. Inthe preferred embodiment, a pair of posts are formed in the side of thewall opposite the channel. These posts are spaced apart and the saddleportion of the clip fits between the posts. The posts act as a positivelocking structure to prevent the clip from being dragged along the wallwhen a card is inserted.

Several raised ribs or bumps are spaced along the bottom and one sidewall of each slot or channel in the bottom card guide. As the electronicmodule or card is inserted into the channel or slot, it contacts andrides on the raised ribs. Since the electronic module or card issomewhat abrasive to the plated card guide, the raised ribs wear witheach insertion or withdrawal. As the raised ribs wear, small amounts ofconductive material are released within the enclosure. However, theamount of the raised rib wear is minimal compared to the wear caused bythe electronic module or card contacting the entire length of thechannel bottom. Furthermore, even when the raised ribs wear there iscreated only a small hole in the Faraday shield which is insufficient toleak emissions. In the preferred embodiment, the top card guide does nothave the wear ribs since the top of the card does not rub against thebottom of the channel in the top card guide and there is no clip toright justify the position of the card against the right wall of thechannel.

Each module or card has an interlock actuator switch assembly whichconnects or disconnects the power to the specific module or card. Themodule or card is inserted with the actuator switch in the "off"position and only after the module or card is inserted is the actuatorswitch moved to the "on" position, thereby providing electrical power tothe module or card. When the module or card is in the proper position,the card or module is affirmatively affixed to the enclosure by screwsor other suitable means. The interlock actuator switch assembly in the"on" position providing power to the module or card, it mechanicallyblocks the securing means or screw so that the module or card can not bephysically removed from the enclosure without disconnecting power to thecard or module by moving the interlock actuator switch assembly to the"off" position.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention will become apparent upon reading thefollowing detailed description and upon reference to the accompanyingdrawings, in which:

FIG. 1 is a perspective view of the enclosure of the present invention.

FIG. 2 is a perspective view of the enclosure of the present inventionwith the panel to the top chamber removed and the electronic modules orcards removed.

FIG. 3 is a top view of the card guide shown cut away in sections toremove redundant material.

FIG. 4 is a detail view of a portion of the bottom card guide showingboth the ribs and anti-static clip.

FIG. 5 is a cross section of the ribs along line 5--5 of FIG. 4.

FIG. 6 is a cross section of the anti-static clip along line 6--6 inFIG. 4.

FIG. 7 is an exploded view of the interlock actuator switch assembly ofthe present invention.

FIG. 8 is a partial view of the interlock assembly in the "off" positionprior to insertion of the module or card into the enclosure.

FIG. 9 is a back perspective view of the interlock assembly showing itsconnection to the card or module.

FIG. 10 is a side view of the interlock assembly in the "off" position.

FIG. 11 is a side view of the interlock assembly in the "on" position.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will be described in detail herein. Itshould be understood, however, that the invention is not intended to belimited to the particular forms disclosed. On the contrary, theapplicant's intention is to cover all modifications, equivalents, andalternatives falling within the spirit and scope of the invention asdefined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is an enclosure for receiving and holding aplurality of electronic modules or cards and which establishes a Faradayshield around the electronic modules or cards to prevent RFI/EMIemissions. In the preferred embodiment, the Faraday shield comprises topand bottom molded plastic card or wave guides which are plated. Itshould be noted that other materials for the card guides could be used,such as a conductive fiber embedded molded plastic. However, if adifferent material is used, additional steps are sometimes required inthe assembly of the card cage, such as if a plastic with conductivefiber is used, the surface conductivity at the points or the locationsof mating with other parts must be enhanced, typically by removing theresin layer or coating. These card or wave guides have a plurality ofmolded channels to receive the modules or cards and an open face gridwith sufficiently large openings for air flow without acoustic noise andyet preventing RFI/EMI emissions. Each card guide has a plurality ofthree-sided or U-shaped channels for receiving the modules or cards. Thechannels of the bottom card guide include a plurality of raised ribsalong at least the bottom surface of the channel. In the preferredembodiment the ribs also extend up one wall of the channel. The raisedribs support the card during insertion and removal to prevent excessivewear in the channel which causes particle contamination and/or a breachof the Faraday shield. Each channel of the bottom card guide alsoincludes an anti-static clip secured in a recess and designed to engagea static strip on each card. Finally, each face plate for each modulewhich forms a portion of the Faraday shield, includes an interlockactuator switch assembly that prevents the electronic module or cardfrom being withdrawn from the enclosure until power to that card isremoved.

Referring now to the drawings, wherein the reference charactersdesignate like or corresponding parts throughout the views, FIG. 1 is aperspective view of an enclosure 10 according to the present invention.Throughout the various figures many detailed portions of the enclosurehave been deleted for the sake of clarity and other parts are shown indiagrammatic form to better illustrate the invention. The enclosure 10has a top chamber or compartment 12, a card cage 14 and an open bottomchamber 16. The card cage 14 is adapted to receive a plurality ofelectronic modules or cards 18a-18k. Of course, the number of electronicmodules or cards involved can be increased or decreased withoutdeparting from the scope of the present invention. Furthermore, thefunction or purpose of the electronic components on any module or carddoes not have any bearing on the present invention. In the preferredembodiment, space is allocated for a power supply module 18a and thenten cards 18b-18k. In the preferred embodiment, the power supply isenclosed in a metal housing and, accordingly, does not add EMI/RFIemission into the card cage 14. At least one fan 20 showndiagrammatically (FIG. 2) is mounted in the top compartment 12 to drawair from the open bottom compartment 16 through the card cage 14, asdescribed in detail hereinafter.

FIG. 2 is a perspective view of the enclosure 10 with the front of thetop chamber 12 removed and the electronic modules or cards 18a-18kremoved. The bulkheads or face plate of each electronic module or cardforms the front of the card cage 14 as is well-known in the field. A fan20 is shown in phantom on the top chamber 12 and can be mounted in anywell-known manner. The top of the card cage 14 is the wave or card guide30 and the bottom is the substantially identical mirror image wave orcard guide 32. In the preferred embodiment, there are some minordifferences between the card guides 30 and 32. The details of thestructure of the card guides 30 and 32 are set forth hereinafter. Thecard cage 14 has a left side 34 and a right side 36 made of sheet metalor similar material. A backplane 38 for connection of the electronicmodules or cards 18a-18k is shown in diagrammatic form only since itforms no part of the present invention. The card cage 14 has a backpanel 40 behind the backplane 38 and is made of sheet metal or similarmaterial. The card cage 14 prevents RFI/EMI emissions from theelectronic components on the modules or cards 18a-18k. The card guides30 and 32 are riveted or otherwise connected to the side walls 34 and 36and the rear panel 40 and the various bulkheads or face plates areassembled in a manner well-known in the field. The card or wave guides30 and 32 are made of molded plastic with electrolyses nickel overelectrolytic copper plating to provide conductivity.

FIG. 3 is a top view of the card or wave guide 32 which is fragmented toshow the invention without unnecessarily illustrating duplicativematerial. In addition, only card or wave guide 32 is shown in detailsince guide 30 is a substantially identical mirror image. Wave guide 32is separated into two portions, 32a and 32b. Portion 32a illustratesthree of the molded channel guides 50, which are adapted to receive oneof the electronic modules or cards 18b through 18k. FIG. 4 shows thedetails of one of the channel guides 50 with clarity. The channel guide50 extends substantially the length of the wave guide 32. There are aplurality of channel guides 50, one for each electronic module 18bthrough 18k. Portion 32b illustrates two additional channel guides 50for holding circuit boards such as the modules or cards 18b through 18k.Additionally, portion 32b supports two channel guides 52, which areadapted to receive a power supply 18a. In the preferred embodiment, thepower supply module 18a is enclosed in metal so that EMI/RFI emissionsare not leaked into the card cage. Accordingly, for practical purposes,the power supply module 18a is isolated and doesn't contribute to theneed for a Faraday shield. Underlying all of the channel guides 50 and52 is grid 54. Since the power supply module 18a is separately shielded,the openings 55 in the grid 54 are larger than the openings 56 in thegrid underlying the modules or cards 18b through 18k. Because there isno source of EMI/RFI emissions adjacent the openings 55, these openingsare slightly larger than openings 56 to allow greater air flow. In apreferred embodiment of the present invention, the size of each of theopenings 55 is approximately 11.75 mm×18.7 mm and 11.9 mm deep.Accordingly, the openings 55 are not considered in the determination ofthe openings 56 as discussed hereinafter. Of course, if a differentcomponent similar to the others was used in module 18a, then theopenings 55 would be the same size as the openings 56 and the channel 52would be the same as channel guides 50. There are a plurality ofapertures or openings 56 forming the portion of the grid 54 subtendingthe modules or cards 18b through 18k. The size and depth of the openings56 are chosen to balance the functional requirement of providing aFaraday shield preventing RFI/EMI emissions and yet permittingsufficient air flow through the card cage to dissipate the heatgenerated by the electronics without causing acoustic noise. Therealization that the grid openings 56 can be relatively large and deepcompared to typical Faraday shield openings and still block RFI/EMIemissions forms a portion of the structure of the Faraday shield cardcage of the present invention. In the preferred embodiment, eachaperture is approximately 11.75 mm×12.3 mm and 11.9 mm deep. In thepreferred embodiment, the openings 56 are rectangular, but theorientation of the rectangular openings is irrelevant. Furthermore, itwill be appreciated by one of ordinary skill in the art that thegeometric shape of the openings can be varied to accommodate any design.Each grid opening 56 has a depth, referred to as the Faraday shieldthickness, that is at least fifty percent (50%) of the diagonal lengthof the maximum opening size. Furthermore, the greatest edge dimension ofany grid opening 56 must be less than one-fourth the wavelength of thehighest frequency of interest. In the preferred embodiment, the openings56 are all uniform, but it is within the scope of the invention toprovide openings of various sizes as long as the above referred tostandards are met.

FIG. 4 is a cut away portion of the wave guide 32 and illustrates indetail a channel guide 50. The channel guide 50 comprises a left wallportion 62 and a right wall portion 64. The left wall portion 62 has alinear or straight portion 66 and an angled portion 68. The right wallportion 64 has a linear or straight portion 70 and an angled portion 72.The left linear wall portion 66 is spaced apart from and substantiallyparallel with the right linear wall portion 70. A bottom member 74extends the entire length of the linear wall portions 66 and 70 andtogether therewith forms a substantially U-shaped channel 76. The frontportion of the bottom member 74 slopes inwardly forming a ramp 78 toavoid having the front edge of the module or card 18 striking a squaredcorner. The ramp 78 together with the right angle portion 68 and theleft angle portion 72 provide a centering structure to direct the moduleor card 18 into the U-shaped channel 76 of the channel guide 50. Thefront surface 80 has a guide hole 114 and a screw hole 116. The moduleor card 18 has a post or projection (FIG. 7) that fits into the guidehole 114 and once the module or card 18 is positioned within the channelguide 50 the front panel is screwed or otherwise fixed in place. Ofcourse, there are a plurality of channel guides 50 across the surface ofgrid 54 of card guide 32. Each channel guide 50 is substantiallyparallel to the other channels. The top card guide 30 is a substantiallyidentical mirror image of card guide 32 and has a plurality ofoppositely facing channels, each of which cooperates with one of thechannel guides 50 to receive and hold a module or card 18.

A recess 86 is formed in the side of the straight wall portion 66 thatfaces into the channel 76. The recess 86 is formed within the firstquarter of the length of the straight wall portion 66. Two posts 90 and92 are formed on the straight wall portion opposite to the recess 86.Additionally, a portion of the straight wall 66 is cut away to show therelationship of the aperture 56 to the grid 54.

An anti-static clip 94 straddles the straight wall portion 66 betweenthe posts 90 and 92. The clip 94 is preferably made from nickel platedcooper and comprises a saddle or base portion 96, a short straight leg98 which lies flat within the recess 86 and a bowed or hook portion 100which extends into the channel 76. The saddle portion 96 fits over thewall 66 between the posts 90 and 92. As the card or module 18 isinserted into the channel 76 a static strip 102 along the bottom portionof the card 18 contacts the hook portion 100 of clip 94 and tends toflatten the hook portion 100 and extends it further along the recess 86.

Because the clip 94 is within the recess 86, the clip is restrained frombeing dragged along the length of the linear wall 66 when the card 18 isbeing inserted or removed. In order to insure that the clip remains inplace the posts 90 and 92 capture the saddle portion 96 and prevent anymovement of the clip 94. The contact between the clip 94 and the card 18aligns or right justifies the position of the card 18 within the channel76 so that the card 18 is properly connected to the backplane and guidehole 114. In the preferred embodiment, the hook portion 100 maintainscontact with the strip 102 which extends the entire length of the card18 and thereby discharges or dissipates any static electric charge. Inthe preferred embodiment the top card guide 30 does not have a recesssince it does not use an anti-static clip. Of course, as an alternativeembodiment, the top card guide has a corresponding structure andanti-static clip which contacts another static strip along the top ofthe card. Accordingly, it is within the scope of the present inventionthat the anti-static clip 94 could be used in both the bottom and bottomcard guide 32 and 30 instead of only in the bottom card guide 32.

In addition, while only one clip 94 is used in the card guide 32 in thepreferred embodiment, additional clips could be used along the length ofthe linear wall 66. Of course, if additional clips are used,corresponding recesses must be formed in the linear side wall andadditional pairs of posts must be formed. It should also be understoodthat the anti-static clip could be attached to the right linear wall ofthe channel with the corresponding necessary structural modifications tothe wall and the placement of the static strip on the opposite side ofthe card.

A rib 104 is molded into the channel 76 along the side of the rightlinear wall 70 facing the channel 76 and along the bottom 74. In thepreferred embodiment, there are a plurality of ribs 104 spaced alongeach channel 76. As the module or card 18 is inserted into the channel76 the bottom of the card 18 rests upon the portion of the ribs 104across the bottom of the channel 76. A rib-like portion 105 extends fromthe right side wall 70 to prevent the card 18 from contacting the rightside wall 70 as the card 18 is initially installed. In addition, as theanti-static clip 94 contacts the card 18, it justifies the position ofthe card 18 by biasing it against the rib 104 along the right side wall70. The ribs 104 protect the bottom 74 and right side wall 70 fromwearing due to contact with the module or card 18. The number of ribsalong each channel is a matter of design choice. In the preferredembodiment, there are no ribs in the top card guide since the module orcard 18 is slightly shorter than the distance between the top and bottomcard guides. Therefore, the top surface of the module or card does notcontact the bottom of the channel in the top card guide and since thereis no anti-static clip biasing the top of card 18 against the channelwall the need for the rib is eliminated.

FIG. 5 is a cross section taken along lines 5--5 of FIG. 4. The U-shapedchannel 76 is positioned over the grid 54. In the preferred embodiment,the length of the bottom portion 74 of the U-shaped channel 76 issubstantially the same length as the web of the grid 54 and the two wallportions 66 and 70 extend slightly over the openings 56 and the grid 54.The depth D of the grid 54 in the preferred embodiment is 11.9 mm. Thesaddle portion 96 of clip 94 straddles the wall 66. As the card 18 isinserted into the channel the hook portion 100 of the clip 94 contactsthe static strip 102 of card 18 contacting the hook 100. Because thecard 18 is now electrically in contact with the channel guide 50 by theclip 94, any static charge build-up on the card will be harmlesslydischarged or dissipated.

FIG. 6 is a cross section along line 6--6 of FIG. 4. The module or card18 is within the U-shaped channel 76. The module or card 18 rides on theribs 104 and accordingly does not contact either the bottom 74 or thewall 70. While the ribs 104 will eventually wear, the amount ofcontaminants released within the enclosure from these ribs is smallcompared to the contaminants released when the card 18 makes contactwith the entire bottom 74 and side wall 70 of the channel 76. Further,even if the platting wears away from the ribs 104, there is only a smallhole in the Faraday shield which is insufficient to leak emissionscompared to the wearing away of the platting from the entire channelwhich leaves a significant hole in the shield.

FIG. 7 illustrates an exploded view of the power actuator switchassembly or interlock 110. A cut away of the module or card 18 isillustrated. The card 18 has a pin locator 112 which, upon insertion ofthe card into the channel guide 50 (FIG. 4), fits into the opening 114in card guide 32. A larger screw hole 116 is also in the card guide 32.Of course, each module or card has a similar structure at the bottom asshown in FIG. 4. A screw 118 fits through the aperture 120 in the frontof the card 18 and into the screw hole 116. The switch 110 has a frontplate 122 which has two protrusions 124 and 126 which extend throughapertures 128 and 130 in the front plate of card 18. A back plate 132receives the two protrusions 124 and 126 from the backside of the frontof the card 18. A switch circuit 140 having a toggle bat 142 cooperateswith back plate 132. The switch 110 has two positions, on and off.

FIG. 8 shows the switch 110 in the off position and the screw 118inserted into the hole 116, which is shown in phantom. In the offposition, switch 110 is entirely below the screw hole 116. In thisposition, the screw 118 can be removed and the entire module or card 18removed from the enclosure without affecting the other cards since thepower to the module is off.

FIG. 9 shows the back side of switch 110. The back plate 132 has areceiving aperture in which the toggle bat 142 from switch circuit 140is mounted on the card 18. In the off position, switch 110 holds thetoggle bat 142 in a downward position thereby keeping switch circuit 140off. As is well known, with switch 140 in the off position, the commonpower is removed from the module or card 18.

FIG. 10 illustrates the switch 110 in the off position with the faceplate below the screw hole 116, the toggle bat 142 downward and theswitch 140 off.

FIG. 11 illustrates the switch 110 in the on position with the faceplate covering the screw 116, the toggle bat 142 is upward and switch140 is on. Accordingly, power is supplied to the card 18.

Accordingly, when power is applied to the card 18, the switch 110 is inthe on position and covers the screw 118. Therefore, it is not possiblefor someone to withdraw the module 18 without removing the screw 118 andthe switch 110 must be moved to the off position in order to remove thescrew 118. Of course, by moving the switch 110 to gain access to thescrew 118, the switch 140 is turned off and power to the card isremoved.

An enclosure for receiving and holding a plurality of electric modulesor cards and providing a Faraday shield to prevent EMI/RFI emissions hasbeen disclosed. The Faraday shield comprises top and bottom moldedplated plastic card guides that are mirror images. The size of theapertures in the card guide are chosen to balance the prevention ofEMI/RFI emissions and the need to have an adequate air flow to dissipateheat without causing acoustic noise. The card guides have a plurality ofchannels for receiving the electronic modules or cards and at least someof these channels have an anti-static clip to dissipate any build up ofstatic charge on the module or card. Furthermore, at least some of thechannels of the card guides have wearable ribs which separate the cardfrom at least the bottom and, in the preferred embodiment, the side wallof the channel, to prevent wear along the entire length of the channel.Finally, each module or card assembly has a mechanical interlockactuator switch assembly which prevents the module or card from beingwithdrawn from the card cage when the power is on. When the power to thecard is on, the interlock assembly blocks access to a retaining screwwhich must be removed to release the module from the card cage. When thepower to the card is off, the interlock assembly is moved, and theretaining screw becomes accessible. Of course, after the screw isremoved, the module or card can be withdrawn from the card cage with thepower to the module being off.

It will be understood that various changes in the details, arrangementsand configurations of the parts and systems which have been describedand illustrated above in order to explain the nature of the presentinvention may be made by those skilled in the art within the principleand scope of the present invention as expressed in the appended claims.

What is claimed is:
 1. A module housed in a card cage comprising acircuit board connected to a front panel having an interlock assemblyfor preventing said module from being removed therefrom when powerprovided by a power supply is being supplied to said circuit board, saidinterlock assembly comprising:fastening means for releasably securingsaid front panel to said card cage; a first switch mounted on saidcircuit board for connecting said power supply to said circuit board,said switch being movable between a first position in which said powersupply is disconnected from said circuit board and a second position inwhich said power supply is connected to said circuit board; and, asecond switch mounted to said front panel and extending through saidfront panel to contact said first switch, said second switch having afirst position which moves said first switch to its first position inwhich said power supply is disconnected from said circuit board and inwhich said fastening means can be accessed releasing said front panelfrom said card cage and allowing removal of said module from said cage,said second switch having a second position which moves said firstswitch to its second position in which said power supply is connected tosaid circuit board and in which said second switch blocks access to saidfastening means preventing the release thereof and the removal of saidmodule from said cage.